S.R. Defining the present

I've been having a really tough time trying to determine what S.R. means by "the present." On the face of it, SR has a different present for each participant. The lines of present, or now slices, are parallel to each participant's x-axis. So at t=0 for the "stationary" participant, the now slice is a horizontal line that represents all distance points on that line are set, by relativity's clock syncing, to t=0. For the "moving" participant, the now slices are slanted and parallel to the x'-axis. All the distance points on the line share the same t' (time dilated) time wrt t.

Somewhere I read that the above rule falls apart if the distance separation between points on a line is great enough but I think that comes from advanced relativity so I don't really know the explanation.

All the points on a now slice share the same nominal sync'd time but do they really share the same present moment? If you saw 3 supernovae come on simultaneously in the sky, there is no shared present between you or them. Shared presents are never seen in the present due to the delay of light. They can only be reconstructed as they happened in the past.

The following STD shows how 3 truly simultaneous supernovae spaced 1 light year apart would be seen on earth turning on 1 year apart. The earth would have shared that simultaneous present 3 years ago.

In the next post we'll throw relative motion into the mix with distance separation to see how the two together define the present in relativity.

Re: S.R. Defining the present

The only "truly simultaneous" events in physics are those that happen at the same place in space (co-located) and at the same time. The rest are all relative and observer/frame dependent, like is "the present". Otherwise, there is always the Movie and another word for a gift... :)

Re: S.R. Defining the present

Sure, I guess the only true present, in the present, is for entangled particles whether they are together or separated, but that too is only between the two particles. I'm talking about a calculated present, between two participants that neither of them was aware of, that happened in the past.

Relativity, on the other hand, tries to create an artificial present through clock synchronization. Every clock in a line is set to the same time so, from one end of the line to the other, any relatively stationary event that is clocked at the same time is deemed to be simultaneous even though they're separated. Two lines of relatively moving clocks will not see the same simultaneity of events. I'm saying that's not the same simultaneity I'm trying to find. Things are not simultaneous in the present for a line of clocks no matter what the time says on their faces. I guess there's face time and real time. However, the clock syncing makes it easier to mark and figure out what was a shared present. My end focus is to figure out if two relative frames aging at the same rate is another definition of present.

Re: S.R. Defining the present

Yes, Einstein found the perfect way to define simultaneity and we never again had to look back.

Make sure that you get the terminology straight. The only frames that I know of that age, are door frames and window frames. I also think you must try and get out of the "aging frame of mind". Rather start to use the correct term like elapsed propertime between two events.

Age is in fact very poorly defined, because (sadly) my feet ages slower than my head for two reasons: they are closer to the center of earth and they often move to and fro relative to my head. So the two-way "twin paradox" applies to them often.

Re: S.R. Defining the present

BurtJordaan wrote:Yes, Einstein found the perfect way to define simultaneity and we never again had to look back.

They didn't have atomic clocks in those days. They didn't even have atoms yet. There are many ways to do timing. It can be synchronous or async or you can depend on the universal accuracy of atomic clocks as the basis of all timing. Then you use light pulses as markers and you just need the accurate duration between them that atomic clocks provide.

Rather start to use the correct term like elapsed propertime between two events.

i.e. aging in the vernacular. Everything ages at the proper time in its own frame and age difference occurs when you compare frames under specific circumstances.

Re: S.R. Defining the present

ralfcis » May 2nd, 2017, 4:35 am wrote:Relativity, on the other hand, tries to create an artificial present through clock synchronization. Every clock in a line is set to the same time so, from one end of the line to the other, any relatively stationary event that is clocked at the same time is deemed to be simultaneous even though they're separated.

Creating a present is not what this is about. The point is to get beyond our invalid assumptions about simultaneity and thus talk about concrete clocks to see what happens. If you change your velocity in the direction of the clocks then assuming those clocks all still read the same time will lead to contradictions.

The only way to overcome these logical contradictions is get rid of our Euclidean idea of space-time as a series of instantaneous snapshot strung together.

ralfcis » May 2nd, 2017, 4:35 am wrote:Sure, I guess the only true present, in the present, is for entangled particles whether they are together or separated, but that too is only between the two particles. I'm talking about a calculated present, between two participants that neither of them was aware of, that happened in the past.

If everything is categorized into past, present, and future, then the present is everything outside the past and future light cones. It is everything at a space-like distance from you, so if d is the spatial distance then the present includes a time interval of length 2d/c (where c is the speed of light). For example, the present at Saturn is a period of time (on average) which is 22 hours long and at Alpha Centauri the present is a period 8.74 years long. These periods are outside your past and future light cones and thus nothing in those periods of time should be called past or future. And these periods do not change when you change your velocity.

ralfcis » May 2nd, 2017, 4:35 am wrote:Two lines of relatively moving clocks will not see the same simultaneity of events. I'm saying that's not the same simultaneity I'm trying to find. Things are not simultaneous in the present for a line of clocks no matter what the time says on their faces.

In sense you are exactly right...

Let's say you have two ships each with a line of synchronized clocks from front to back and one passes the other at a relativistic speed. Then for each the clocks on their own ship remain synchronized. It is the clocks on the other ship which they conclude are not synchronized. And that is where you are correct to think that this synchronization does not define the present. But the correct conclusion is that the idea of the present as a snapshot in time must be discarded. According to the above explanation, all the clocks in our example are in the present because at a distance the present is a time interval which includes the times they see on the clocks of the other ship (no matter how fast the other ship is going).

This is not the solution because remember that for each of the two ships the clocks on their own ship remain synchronized. This is not about clocks not reading the right time. But it is about needing to redefine what is meant by the present. That part is correct.

Re: S.R. Defining the present

Sorry mitchellmckain, can I respond to your post in 5 months. I'm just trying to write down what's in my backlog before I forget it all. Also my threads get shut down if I take too long completing them.

Re: S.R. Defining the present

There are many pieces to the puzzle of defining the present in relativity. The pieces are:

distance separationrelative velocityslope of present linesslope of same age linesspeed of light time marker signal pulsestwo participants relation to a common moving or stationary reference framerelative time dilation coordinate values at line intersections

Yes it's a difficult sleet storm to trudge through especially with the problem of establishing a common terminology of what the above terms mean.

Relativity doesn't condone the idea of a universal present with everyone in the universe sharing the same calculated slice of the present no matter where they are. But in truth, most relative velocities between most participants are far enough away from the speed of light so there is a slightly fuzzy universal present. But we're not talking about that, we're talking about the precise relativistic definition of the present.

Re: S.R. Defining the present

distance separationrelative velocityslope of present linesslope of same age linesspeed of light time marker signal pulsestwo participants relation to a common moving or stationary reference framerelative time dilation coordinate values at line intersections

Since the actual present isn't a line, any lines which you have marked as "present" is a mathematical artifice, or at least just one of many lines which are all a part of the actual present. Not sure what is meant by "same age lines." In any case, until you master the relativity of simultaneity you will never understand relativity.

Here is a good example for getting a handle on this. Suppose ship A is traving at 86.6% of the speed of light with respect to B, and his path takes him through a tunnel, where B is waiting, just a little more than half the length of his ship. Lorentz Contraction means A is 1/2 as long in the direction of motion according to B, so B plans to slam down two doors in order to trap A in the tunnel. What happens?

Well according to A the front door slams down first and so either A has to stop or he crashes into the door. Either way, the door behind him only shuts when the tail of his ship passes through it. This means that if A does try to stop that fast it will be as bad as crashing into the door, because neither force nor message can travel to the ends of his ship in order to make all of it stop before that second door slams down.

ralfcis » May 8th, 2017, 11:10 am wrote:Relativity doesn't condone the idea of a universal present with everyone in the universe sharing the same calculated slice of the present no matter where they are.

Correct!

ralfcis » May 8th, 2017, 11:10 am wrote:But in truth, most relative velocities between most participants are far enough away from the speed of light so there is a slightly fuzzy universal present. But we're not talking about that, we're talking about the precise relativistic definition of the present.

Incorrect. The definition of the present has absolutely nothing to do with relative velocities. At least, the definition I gave above is the definition which doesn't change with velocity. That 8.74 period which is the present at Alpha Centauri doesn't change with velocity, and I don't just mean the length doesn't change but the actual period of time, doesn't change, so this definition of the present is invariant under the change of velocity.

Re: S.R. Defining the present

I forgot the most important piece: reciprocity for time dilation, aging, televised aging and present time. But more on this later, for now let's start with the basics.

In this STD there is Bob's stationary vertical axis compared to both a light signal that bounces back to Bob after 4 years and Bob's stationary friend Charlie 5 light years away. The horizontal dotted lines are Bob's present lines extending out into space.

Bob sends out a light pulse at t=0. That point is a very high level 4 present (no such thing as a high level 4 present in relativity but I will show there are different degrees of present so bear with me). Bob is co-located with the light pulse and shares that present, in the present. As soon as you bring in distance separation, the present must be calculated to find out what was the present then, not what is the present now.

Since Charlie is separated from Bob, he shares a level 3 present with him. His shared present with Bob is not shared in the present, it must be calculated 5 years later. This can be done in 2 ways. First, pre-sync Charlie's clock with Bob's network clock which is how relativity does it. Another way is Charlie sends pulses out to Bob with Charlie's clock reading. The time Bob reads from Charlie when Bob has aged 5 years is zeroed much like a stopwatch at the starting gun. Subsequent clock readings from Charlie will have this initial value subtracted from them. The universal independent accuracy of atomic clocks makes this method possible.

When Bob has aged two years, light has traveled 2 years. So light and Bob are traveling through time (aging) at the same rate. Charlie also shares the same aging rate along the same horizontal dotted line. Bob's relationship with light is downgraded to a level 3.5 present. It's no longer co-located with Bob but it's light so we don't really need to verify that it has reached the 2 light year mark in 2 Bob years.

Charlie is not light so he does send a signal back to Bob at the two year mark as a courtesy to verify their clocks are sync'd. The light signal Charlie would send back to Bob would reach him at Bob t=7. It would confirm that the zeroed clock reading for Charlie would show that 5 years ago, both Bob and Charlie had aged 2 years at the same rate of 1 yr/yr. Not only had they aged the same but their clocks have effectively the same time. They have aged the same at the same time; there is no time dilation. The clocks along a dotted line all have the same face time and Charlie's present lines are horizontal and overlap Bob's present lines. None of this will not be true once relative velocity is brought into the picture.

Televised aging rate (which I also forgot to mention as a piece of the puzzle) is represented by the diagonal dashed lines between Bob and Charlie between t=0 and t=1. Bob sees Charlie age 1 yr per Bob year and Charlie will see Bob age 1 yr per Charlie year. The televised signal between them will reciprocally show the images flow at the same rate of time (normal proper rate of time flow) as their own. Once relative velocity comes into play, the reciprocal images will come in slow motion or fast forward depending on the direction of the relative velocity.

It's important to note the televised aging rate is not the same thing as the aging rate. I can only clarify this statement once we get into the discussion of how relative velocity changes things. In this example, the televised aging rate is the same as the actual aging rate of 1yr/yr.

These minutiae all seem incredibly tedious but it sets up how all the puzzle pieces making up the present will begin to diverge from each other once relative velocity is brought into the picture. Once we can account for the divergence, we can generate a true picture of what was the present with all the pieces in play.

Please if you have any objections to the terminology, now's the time to correct them. I'm not sure relativity has terminology for these puzzle pieces.

Tomorrow I'll introduce an STD that shows how the pieces begin to diverge when relative velocity comes into play. Things will get very complicated after that.

Re: S.R. Defining the present

Just no time to keep this thread going but I am doing the math in the background and it's open to so much interpretation that I'm worried this thread will degenerate into philosophy and then get shut down.

So here's what I'm understanding of the present so far (and will present the math at the end). It looks like the present is an individual bubble surrounding each of us. The farther away from your bubble, the more past of everything else you see. (The stars you see now are how they looked way in the past.) Even if everything and everyone around you is in the same frame, you are not all seeing the same present even though you're all sharing the same present. So if a bunch of bubbles are all moving at zero relative velocity strung out all over space, they don't coalesce, they remain separate but strung together by a calculated common present.

Is reality what you share (calculated) or what you see (subject to the speed of light delay)? If the sun disappeared, our shared present is that we don't know the sun is gone but for 8 min our present reality is that the sun is still there. This theme of apparent present and calculated present permeates relativity but which of these is actually defined as reality?

There are more facets to this question. If the present is a small bubble and length contraction is only in the direction of motion and reciprocity exists between two participants engaged in a relative velocity, then any effects we theorize about what we would see of a ship at .6c should be the same as that ship sees of us. Currently SR says we would see very localized dilation/contraction of the ship but the ship would see the same effects occurring to us on the scale of the entire universe.

But is this really true? I'll show you my math later but for now let's just put our finger to the wind. The farther anything is away from your bubble, the smaller it looks and the slower it looks. These effects are just illusory perspective but we can calculate the underlying reality.

Let's take this to the extreme and explore where the theory comes from that the CMB is an absolute frame of reference. It is both all around us and is equally distant from everything in the universe. It is as far in the past as we can see yet we can calculate a shared present with it as well (based on whether expansion of the universe has a relative velocity to it or not). It is so far away that time appears to stand still, 0 velocity relative to everything. This is apparently true but not really true. The distance from the CMB means a ship traveling .6c relative to us will not see the universe any more contracted or time dilated than we would see the same universe around the ship.

Consider the popular version of relativity that states you can't tell if the universe is rushing past you or you're rushing past the universe. That concept, as stated, requires an implied anchor point that is outside the universe. It's like the universe is a record put on a record player and the needle is fixed to the record player while the record spins underneath it. It's like the LHC is spun around stationary protons. These concepts may be valid mathematically, but they are not valid physically. You can never consider the mass of the universe spinning underneath you in the same way you can't consider hidden stagehands moving the scenery around you like we are all actually paralyzed in space. Those concepts are not physical reality, one can always trace back into the past on who experienced acceleration to get moving.

You don't need the idea that either frame can be considered stationary, all that counts is the RELATIVE motion which can be depicted any way you want in an STD. If Alice is moving, you don't ever have to consider her as the stationary frame, that is all embedded in the STD, you just need to see it.

Here is an STD at .8c of what each participant sees of the other. The view is identical, it's just the coordinate systems that make it look different. Just like Bob sees Alice in square coordinates, Alice is also seeing Bob in square coordinates but Bob looking back at himself through Alice's eyes would see the rhombic coordinates.

The dotted lines represent the present from each perspective. When Bob is 3, he sees Alice at 1.8 her time at 2.4 ly. When Alice is 3, she sees Bob at 1.8 his time at 2.4 ly her spatial coordinate. From Bob's perspective they may age at the same rate but they do that at 2 different present times. For example, Bob is 3 at t=3 but Alice is 3 when t=5. But what if there was a perspective from which both Bob and Alice are 3 at the same time and age at the same rate. Or what if Alice and Bob had a relative velocity of .8c but had no reciprocal time dilation between them and aged at the same rate at the same present time from both their perspectives. The STD's depicting these will be developed in the next post.

Hint: a reference frame doesn't need to be stationary nor does it need to be tied to one of the participants.

Re: S.R. Defining the present

It appears there's a break in my workload. But before I continue, I have a question to ask. Hopefully this thread doesn't get shut down because of it as there is a long way to go before I can wrap this up.

So far we've established the more co-located the two participants are, the more they share the present in the present (highest level of present). No matter what their relative velocity, relativistic effects such as time dilation and length contraction disappear when they're co-located. Even if they speed past each other, the moment they're neither approaching or separating is the moment that they're in the same common frame, sharing the same present in the present. They share the same proper distance and proper time at that moment.

Outside of this close range, the farther they are apart, the greater the influence relative velocity exerts. The parallel lines connecting their same ages move from horizontal to a 45 degree limit. The lines of present also separate from each other and a common present is no longer shared between the participants. Although they age at the same rate, the separation of the lines of present mean each participant sees the other reach the same age at a later time. This "paradox" of reciprocity is what ensures two constant relative velocity frames will age at the same rate and there will be no age difference so long as they remain in constant velocity.

Popular relativity depicts a ship moving at .8c will see the universe warped and contracted around it and we would just see the ship itself contracted. But is this really true? Things close to the ship will be in the same frame so there are no relativistic effects for co-located participants. Things far from the ship will also not be length contracted for different reasons. i.e.

a) The speed of light will delay any relativistic distortions from being seen in the present.b) Length contraction is in the direction of travel so things on the side won't look contracted. c) But most importantly, reciprocity will prevent the ship from seeing things any differently from how we see things.

I don't believe the universe would look any different to us just because a ship is speeding away from us. If it doesn't look any different to us, it reciprocally can't look any different to the ship. It seems to me the background universe is not fixed to either frame. That's my question, is the background universe only part of our frame and my answer differs from what popular relativity states. Any opinions?

Re: S.R. Defining the present

Ralf, your problem is a mangling of what "sees, "looks like" and relativity's "observe" means. The latter means 'scientifically measure', which may include 'look and see', but using the principles of relativity in the process. For example, measuring relative length contraction requires using the relativity of simultaneity. But you know all this.

The universe surely 'looks different' to observers moving relative to each other, but your statements are so vaguely defined that IMO, no sensible answer can be given to your question.

Re: S.R. Defining the present

Yes difficult to agree on terms. For example, if your friend is 100 yds away he looks shorter. You can take out your ruler and hold it up to him and measure he looks shorter and keep a record of the measurement even though you know he isn't really shorter. Once you take into account perspective, you can use your measurements to calculate his true height.

Similarly, once you apply relativity, you can use your measurements to calculate what was happening at a moment in the past which is usually not directly measurable in the present. The two participants are not directly aware of their reciprocity but they are aware of relativity which says each sees the same length contraction and time dilation in the other. Under certain circumstances, the reciprocal time dilation looks like it becomes the persistent reality of age difference while length contraction never persists even though some argue its record does.

Relativity throws in another curve ball when you consider a televised return journey from Alice. Her TV picture will look in slow motion (time slowed down) going away and in fast motion (time sped up) coming home yet for the entire journey her time was dilated (slowed). Also, for the entire journey, Alice aged at the same rate as Bob except during the turnaround which gave her slowed aging overall.

So it looks as though time slows and speeds up according to the TV signal (measurement) but actually it's slowed relative to Bob (calculated) while remaining normal speed relative to Alice. Aging, also related to time, follows a separate pattern where for both halves of the journey they age at the same rate and at turnaround Bob gets a lump sum age bump (not visible until they meet). All this is going on while no head to head clock comparisons can be made in the present moment due to the separation between the clocks.

It gets even worse at the turn around point when Alice's perspective of Bob's present swings wildly from including his past to including a future that hasn't happened to him yet. For you this is all settled science and makes perfect sense and my questions are impertinent. The fact that all this is "counter-intuitive" gives even more credence to the argument that it should be just accepted and not questioned because a lot of brilliant minds and scientific evidence can't be wrong.

Nevertheless, you did answer my question. According to relativity, reciprocity does not include each participant's view of the background. The moving person will therefore be able to tell he's the one moving just by looking out his window. I tried to propose how relativity could preserve perfect reciprocity that includes the background view but this seems either incorrect or unimportant.

Anyway, my next post will discuss different perspectives of relative velocity and how physical differences in reciprocity reconcile with the inescapable need of mathematical equivalence for any relative velocity scenario.

Re: S.R. Defining the present

Until very recently I held the wrong belief that if I could prove that only one of the participants was actually moving, there was no need to do the reciprocal analysis of the other participant being the moving frame. If no reverse analysis was required, then only the moving guy was time dilating and aging slower than the stationary guy. This is an argument used by those who use the CMB as an absolute frame. As an added bonus, it's so equally far away from everything that its extreme spacetime path length would make it look to move so slowly off in the distance that it could be considered a near absolute zero velocity reference frame. I also didn't buy that it was realistic to consider a ship stationary relative to a universe speeding past it when it was the ship that had been accelerated to the relative velocity, not vice versa. As was discussed in the last post, the ship moving through space would see the relativistic effects on the universe around it to be quite different than how Bob back on earth would be seeing the same universe. (Whether or not this is true will prove irrelevant to the discussion.)

Similarly I saw no way that powering up the LHC to accelerate some protons could be looked upon as being equivalent to spinning the LHC and the entire earth around stationary protons. The protons had to be the moving frame and the only frame that was experiencing time dilation and slower aging. Mass was the source of the asymmetry and, if great enough, its movement would have a negligible share of the relative velocity with less massive objects.

I can now see this is all not true. I see the source of this misunderstanding on the embedded concept of a stationary and a moving frame. There is no need for any of this as was seen in the perfect symmetry of Bob's view of Alice and Alice's view of Bob.

The circled times in the STD show that for Bob at t=3 he sees Alice at t'=1.8 at x=2.4. They correspond exactly to what Alice sees at t'=3 of Bob at t=1.8 at x'=2.4. (The only difference is that the length of the rhombic lines is 2.13437 times the length of the cartesian lines due to pythagoras.) No need for any reverse analysis where each participant gets a turn using the square coordinates; the view from the rhombic coordinates is identical.

But this revelation breeds a new problem. It means that any depiction of .8c relative velocity is identical to any other. This means two ships going towards or away from earth at .5c (yielding .8c relative velocity to each other using the velocity combination law) is identical to Bob being stuck on earth and Alice taking off at .8c or the entire universe being accelerated past Alice until it reached a constant relative velocity to her of .8c or the entire universe being accelerated at .5c past a floating Bob and an Alice going at .5c relative to the universe. All these scenarios are shown in the STD below:

I'll get into this in detail in the next post but for now let's just consider the .5c lines forming a V around the vertical timeline that represents the earth. Bob and Alice are heading away from it at .5c. There is no way to depict them as stationary relative to a single universe moving relative to them. My point for now is, the old way of stationary and moving frame analysis is not valid. I'm saying all the .8c relative velocity scenarios mentioned can be treated as equivalent. So I can use any one of them to further my quest to determine the definition of present in S.R. I've run out of time for now.

Re: S.R. Defining the present

ralfcis » 30 May 2017, 13:38 wrote:Nevertheless, you did answer my question. According to relativity, reciprocity does not include each participant's view of the background. The moving person will therefore be able to tell he's the one moving just by looking out his window.

No, the 'moving' person cannot look out the window and tell that he is 'moving', except in a relative sense, but as you know, that that's not "moving".

I haven't got the time to read through all this, so there may be other misconceptions lurking in the mass of words.

Re: S.R. Defining the present

There are two sections about how we'd see a taxi length contract as it passes us (at 10:52) and how the taxi driver would see city buildings curve in as it passed them (at 16:00). Although both the taxi and us would look reciprocally contracted to each other, the city buildings do not curve for us as they do for the taxi which is a break in the reciprocity. He would see this curving outside his window so he would know he was moving relative to us and to his background.

Of course the farther away the buildings, the less noticeable the difference in reciprocity would be. Also, the moment the taxi was at its closest approach to us would be the moment both of us would be in the same frame and there would be no length contraction or time dilation in that moment. We would be aging at the same rate in the same proper time at the same present moment. As Jorrie said, "The only "truly simultaneous" events in physics are those that happen at the same place in space (co-located) and at the same time." .

This break in reciprocity makes no difference to the fact that when Alice travels 4 ly away at .8c that she will calculate that from her present line, Bob has only aged 1.8 yrs moving 2.4 ly away from Alice and reciprocally, when Bob has aged 3 yrs, from his present line Alice has only aged 1.8 yrs and is 2.4 ly away from Bob. All this can be read from this STD.

So long as this interrelationship between time dilation and aging remains reciprocal, there is no aging difference between the two. Yes this is a rather arbitrary rule in that neither can trust what they see of each other until they compare notes in the future. At that point they will reject the illusory paradox that they both saw each other the same amount of years younger for the same amount of proper time that had passed on their clocks. I prefer a way that they can determine age difference independently which is the way I'm outlining on the personal theories forum.

The next post will show the relationship between horizontal age lines and skewed present lines and horizontal present lines and skewed age lines.

Re: S.R. Defining the present

Hello. "Temporal Dilation" is nothing more than an ILLUSION created by motion and/or environmental circumstances. The fantasy that twins would age differently if one travelled at light speed away from the other is based on the Einsteinian 'thought experiment' in Special Relativity where an Observer on a train accelerating to C PERCEIVES a Clock's hands to move slower and stop as C is approached and attained. Einstein's 'thought experiment,' however, fails to take into account the basic Physics of one object moving away from another at an accelerating rate into hypervelocities. The Information from the clock being carried by photons can no longer reach the train-board Observer once the Observer's platform (the train) attains the same speed as the photons carrying the Information. That does NOT mean Time Absolute (tA) has stopped ... merely that the physical movement of the Train/Observer and photons.

Re: S.R. Defining the present

A few posts back I wrote, "But what if there was a perspective from which both Bob and Alice are the same age at the same time and age at the same rate". The central part of the following STD depicts that. The earth is the vertical axis and represents a stationary frame that is common to both Bob and Alice leaving earth in opposite directions at .5c relative to earth and at .8c relative to each other (by the velocity combo law).

Notice at t=3.46 earth time, the same-age dotted line is horizontal and Bob and Alice are both 3. The equal spacing between the parallel age lines shows Bob and Alice are aging at the same rate. Earth's horizontal present line at t=3.46 is the same as the horizontal age line of t'=3 for both Alice and Bob. However, Alice and Bob's present lines (unfortunately not drawn) are parallel to their .5c x'-axes and go from age 3 to age 1.8 on each (not horizontal). They see each other reciprocally time dilating at t=Yt' where Y=5/3 while they are aging at the same rate. Their time dilation relative to the Earth frame is also reciprocal but is based on .5c Y=1.1547.

The perspective that both are leaving the earth at .5c is equivalent to the perspective (only between Alice and Bob) that Alice leaves Bob back on earth at .8c. Notice how the same-age dashed line between Alice and Bob is slanted away from horizontal. Also the common reference frame between them is now at .5c. Alice and Bob are both moving .5c relative to that and .8c relative to each other. It's exactly the same STD only skewed. Ran out of time.

Re: S.R. Defining the present

The goal of this thread is to fit all the pieces together to solve the puzzle of how SR defines the present. The last post fit 2 of the pieces together. It defined a reference frame around which the half-combination velocity adds up to the relative velocity between the two participants such that its lines of present include both participants at the same age (same-age lines between the participants).

The formula for the half-combination velocity (x) is

v=2x/(1+x2/c2)

So for v=.8c, x=c/2. For v=.6c. x=c/3

In order to find the point where the line of present intersects the reference frame, you take the shared age of the 2 participants and multiply by gamma of the half-combo velocity. So for v=.8c, x=.5c, Y=1.1547 (of .5c). If both participants are 3, then the intersection of that line to the reference frame is 3.46 yrs. At 3.46 yrs the reference frame will include in its present line that both participants have aged 3 yrs simultaneously.

Of course if you choose earth as the reference frame, it cannot see Bob and Alice both at age 3 simultaneously in a line of present. Alice will have aged 3 yrs when Bob's line of present is at 5 yrs and vice versa. Bob's present line is horizontal while Alice's separate present line is slanted between Bob=3 and Alice =5 yrs. The same-age lines between the two have a different slant as well.

Aside from showing how same-age lines and lines of present are different (except from the perspective of the half-combo velocity reference frame) and how reciprocity is arbitrarily used to cancel out age difference during periods of constant relative velocity, the next post will show that relativity ascribes the source of any aging difference to come exclusively from a change in velocity. The faster the period of change, the more sudden the age difference between the two.

For other reasons which I've described at length in past threads, relativity dictates the age difference cannot be ratified until the spacetime path ends at the two participants re-uniting. But the fact that the age difference can be ratified when Alice comes to a stop in space and the light from the stop re-unites with Bob, the exact same logic must mean only the light from a turnaround must reach Bob and not Alice herself. I mean relativity dictates all the aging difference occurs outside of constant relative velocity (which is the turnaround) so why would you need to wait for Alice to re-unite with Bob? You wouldn't, just like you don't need to wait for Alice if she stopped in space to determine age difference.

At some later post I will show that without any turnaround or acceleration that merely starting at a distance from Bob, Alice will age less than Bob. This means no age difference if Alice takes off from earth to another planet, but if she comes from another planet, she will age less. After verifying here how relativity deals with age difference, I'll be returning to the ralfativity thread in personal theories to spend months on continuing to show there's a better way.

Re: S.R. Defining the present

So far we've only integrated 2 pieces of the puzzle: same-age lines with lines of present. There is only 1 perspective from where the two participants are the same age simultaneously in the present. Otherwise they are the same age at different times from each others clocks. So what does that mean?

It could mean I was 3 in 1960 and you were 3 in 1980 but that's not what it means here as the two participants are indeed the same age at different times but they started at the same age. Their age is tied to the same passage of time so if they started at age 0 at the same time they cannot be the same age at different times. Unless of course their clocks were off.

But this is also not true for relativity. Everyone's clock, no matter the constant relative velocity of their frame to any other frame, measures the same rate of time flow and they all age at that same rate within their individual frames. The clocks run perfectly the same in their own frames. So how can they possibly show differences when frames are compared or twins end up aging differently?

If their clocks are universally accurate atomic clocks, when 3 yrs have ticked off on each of these clocks, the participants have aged 3 years period. The aging is a form of universal present that transcends the individual lines of present that join frame's clocks at points that correspond to the formula t=Yt'. The universal aging remains intact so long as all frames remain in constant relative motion. When a guy is 3 in one frame, it is guaranteed he's 3 in all others in constant motion. This makes it very attractive to abandon the use of time dilation to label age. At .8c, Bob is not 5 when Alice is 3, Bob is 3 when Alice is 3. From there we can work out why and under what circumstances do Bob and Alice break from aging at the same universal rate and how does the difference in aging manifest itself.

This new tack does not try to calculate what Alice's age is in Bob's present. When Alice sends a signal to Bob on her 1st year, he will receive it in his 3rd at .8c. You could calculate that in the 4/3 yrs it takes Bob to receive that signal, Alice will be 1.8yrs sharing the same present with Bob having aged 3 yrs. But I say the only measurable facts at .8c known are that for any age a signal starts at, it will be received a factor of 3 yrs later. This is true for Bob or Alice sending signals.

Using this method, you can calculate Bob's age when Alice ages 2 more years to become 3. Bob will receive Alice's signal that she is 3 when he is 9. Bob will have received 2 more signals from Alice and knowing each signal is received every 3 of his years confirms he has aged 3 yrs when Alice has aged 3 yrs. I know, this will take some time to get used to if you are too accustomed to look at relativity from the perspective of clock readings as opposed to age. For one thing there is no reason to invoke reciprocity to dismiss the twin paradox. The clock readings are not consulted, only the age signals between the frames are and they are offset by the speed of light delay. You are free now to misinterpret what I'm saying, overlay what you think I'm saying and then raise your objections.

Re: S.R. Defining the present

In the regular world, time passage and aging are synonymous. This is still true for proper time within frames so everyone, no matter the separation or constant relative velocity to others, will age at the same rate. But at .8c if Alice and Bob were asked how much time the other took to age 3 yrs, they would both say the other only took 1.8 yrs to do so. Plus, depending on which intermediate reference frame was chosen for them, they could reach the same age simultaneously or at different times from the reference frame perspective.

For example if Bob or Alice are the reference frame, then Bob and Alice would have reached the age of 3 when one was 3 and the other 5. But they still would agree the elapsed time on the other's clock to reach 3 was only 1.8 yrs. If the half-combo relative velocity reference frame was chosen, then they would both achieve 3 simultaneously from the perspective of that reference frame. It may be at the same moment but they'd still see the same elapsed time of 1.8 yrs for the other to age 3 yrs.

Although the half-combo relative velocity reference frames re-align simultaneous aging with the specific present of that reference frame, I do not presently see a way to join all the same proper time aging into one universal present based on the same elapsed aging. I also don't know if relativity has a different explanation of how to reconcile elapsed time with aging other than to say the different perspectives on each others time dilation cancels out and by default there can be no age difference between the two participants. Sounds like a band-aid to me: there are no paradoxes in relativity and if you think you've found one then you haven't understood there are none.

Re: S.R. Defining the present

Ok as I got into scenarios of Alice stopping in space or Alice returning to Bob I was finally able to piece together how relativity determines age difference. Relativity does not determine age difference using same-age lines or speed of light messaging or TV image speed comparisons between the participants; it uses alignment of the lines of present between the two. When Alice stops in space her slanted lines of present become horizontal and align with those of the stationary frame. When Alice passes close to Bob, the present lines align by virtue of being inside a point. But let's begin with baby-steps:

The 2 STD's are reprinted here for convenience. The top one is a wide angle view and the bottom one has more resolution.

Let's say Alice's destination is a planet 4 light years away. This planet is part of the earth-galactic consortium whose clocks are all set to earth time. If Alice lived on this planet, her time line would be vertical and her lines of present would be horizontal coinciding with Earth's lines of present. So if Bob aged 5 yrs on earth, Alice would age 5 yrs on her planet. On the fifth year she would send a message back to earth, it would take 4 yrs to get there so Bob would be 9 seeing Alice when she was 5.

You'd be surprised at how many people think this speed of light delay is time dilation since Alice appears younger than Bob so her clock must have been slowed. Others interpret it as age difference due to spatial separation and she will now always be 4 yrs younger than Bob. Others see it as proof that past, present and future all co-exist simultaneously, that we're somehow able to peer into Alice's past which she is currently experiencing in our present.

Seems laughable until you change the scenario where instead of Alice living on the planet, she decides to travel to the planet and reach it when Bob is 5. At that exact point in spacetime where she sent her message to Bob, she is now only 3 according to her watch while Earth-time on the planet is 5. This extra 2 yr discrepancy is where the above relativity effects kick in. The explanation has many moving parts. More tomorrow.

Re: S.R. Defining the present

In S.R., the present is analyzed in 3 different ways: what the present was in the past, what the present is now (whether known and shared by 2 co-located participants or unknown but assumed between 2 separated participants) and what the present will be assuming there are no changes in trajectories.

So in the previous example, when Alice is on the planet 4 ly away, it should be safe to surmise if she was 5 and it takes 4 yrs for her signal to get to earth, she will also age 4 yrs to share an assumed present of both her and Bob being 9. I don't think there's a way to actually verify this in the present as any attempt would require sending light delayed signals. As a result, I feel SR has a real reluctance to make any predictions of what the present will be in the future.

A good example of why there's a reluctance was discussed in past threads when comparing the aging process of Alice and Bob's stationary perspective of each other at .6c. The reverse analysis of Alice going out 4ly and then returning is supposedly equivalent to Alice being stationary for 4 yrs and then catching up to Bob at .8824c. Relativity allows the age difference to be known only at the end of the spacetime path because if it didn't, Alice would age 1 yr less per direction if she's the moving frame while Bob would age less, while Alice was stationary, and then Alice would have to overcome this discrepancy once she took off in order to have the same final age difference in both analyses. This inconsistency forces SR to prohibit any predictions of a shared present during the spacetime path.

It does not even allow determination of the present between 2 co-located objects during the spacetime path. If Bob had a network of sync'd clocks spread through space, reading any of these clocks at close range would agree with the reading of Bob's pocket watch back on earth. If Alice was passing one of these space clocks, the moment she was neither approaching or separating from it would be the moment she would be sharing the same present with Bob's entire clock network and the two could compare clocks. This would allow them to agree on the age difference from Bob's perspective. The reciprocal age difference from Alice's perspective would not be measured but it could be surmised from relativistic principles as cancelling out the age difference as seen from Bob's perspective. Since all this will work itself out at the end of the spacetime path, why can't the measurements from Bob's persepctive be valid only from Bob's perspective?

Let's see how all this relates to our present example at .8c and Alice =3yrs at the 4ly mark. Remember, we are trying to understand how SR defines the present. You will notice a dashed line joining Alice=3 and Bob=1. This is the light signal sent from Bob at his 1 yr mark and it will reach Alice at her 3yr mark. It takes 4 yrs to travel so from Bob's perspective, he is 4+1=5 when Alice is 3. Due to reciprocity, SR does not allow us to say the age difference is 2 because the lines of present between Bob and Alice's perspectives are not aligned into a shared present. However, the information is available on the planet's Earth-sync'd clock as she passes by it: she is 3 from both her and Bob's perspective when Bob is 5.

The dotted line from Alice=3 to Bob=1.8 is Alice's time dilated perspective of Bob. (The horizontal dotted line from Bob=3 to Alice = 1.8 is the reciprocal view of Bob=3.) Bob's present time of 5 at Alice=3 is the addition of Alice's time dilated perspective of Bob (1.8 yrs) and the relativity of simultaneity (vx/c2 = .8 * 4 = 3.2 yrs. This is like a big savings account of time accumulating as Alice and Bob separate and it's cashed in if Alice comes to a stop and her line of present swings horizontal to align with Bob's. At that point, according to SR, Alice is officially 2 yrs younger than Bob from both perspectives. This time is a lump sum settlement and cannot be revoked even if Alice quickly returns to separating from Bob at .8c. Even if we could make Alice's stop and re-start instantaneous so you couldn't even tell from the STD that it ever happened, the age difference is real in one case and reciprocally cancelled in the other. I'm just reporting what I think SR says and don't necessarily agree.

In the next installment we'll look upon how this lump sum age difference differs between Alice separating from Bob (where the lump sum is accumulated) and Alice approaching the planet (where the lump sum just appears at the start of the journey). This is where popular SR gets the idea that because Alice's present lines can include Bob's future or past events that the past, present and future all co-exist in reality as opposed to just in perspective.

Re: S.R. Defining the present

Ok all my dominoes are set up and I'm about to put the whole thing in motion. I had hoped someone would have been able to see what the end result will be but that's ok, I'll take the time to make it painfully clear. I gotta give thanks for this forum because I've been given a lot of leeway to present my argument. It's probably because I've worn everyone down with these long posts but I know the other physics forum wouldn't have tolerated my shenanigans for 1 second. (I'm still banished just for asking a question.) I'm very excited because the end of this thread is near if only work would give me the time to write it all out.

Re: S.R. Defining the present

Before I continue with my tome, I'd like any relativists out there to answer this question:

If Charlie passes near a planet 4 ly away and

a) all the clocks there are sync'd to earth time and b) he sync's his clock to earth time as he passes andc) his relative velocity to earth is .8c thend) as he passes earth, his clock will say 3 yrs have passed while the earth clock will say 5.e) the spacetime pah has a valid start and stop so this is not reciprocal time dilation but real age difference.

True or not?

If true, which I know it is, then how come the reverse is NOT true? If Alice takes off from earth and passes the far planet, S.R. says no age difference has occurred; it's all reciprocal time dilation that can only be resolved as age difference if Alice either relatively stops in space or co-locates with the earth frame.

Why would co-location with an earth sync'd clock on a distant planet not count as co-location with earth (taking into account the time it would take earth to verify Alice has co-located with the distant planet's earth clock)?

If the two scenarios are the same, except for the signal delay, wouldn't that mean that any earth clock, in Alice's path, establishes age difference in increments along the entire path? Again, this age difference can only be established after the signals from each clock reach earth and post processing takes place.

I can't wait to see if you can explain your way out of this one because neither answer is compatible with relativistic doctrine.